So I have a Garmin 405cx and a GSC 10. The sensor is supposed to pick up cadence and speed (wheel speed). But when I use on my cycling trainer I only get cadence and not speed, and consequently no distance.
So the question is, am I missing a setting so that my garmin can pick up speed (non-gps) so that my workout metrics are complete?

Since the cadence is transmitting OK, it's probably a wheel magnet positioning problem.

Use the test/reset button on the GSC-10. It will blink green for each pass of the spoke magnet, and red for each pass of the crankarm magnet. Here's the Garmin setup pdf link. It stops blinking after 60 passes.

I find that the position of my (non-Garmin) spoke magnet is critical, both in spacing to the sensor, and alignment along the sensor arm.